1. Field of the Invention
The present invention is in the field of capacitance diaphragm gauges which measure pressure based on the deflection of a diaphragm.
2. Description of the Related Art
Absolute capacitance diaphragm gauges (CDGs) measure pressure by sensing the capacitance change associated with deflection of a diaphragm whereby one side of the diaphragm (“the Px side”) is exposed to the pressure to be measured (Px) and the other side of the diaphragm is exposed to a sealed reference vacuum cavity in which an ultrahigh vacuum (e.g., less than 10−9 Torr) has been created prior to the sealing of the reference cavity.
The CDG measures capacitance between a diaphragm and one or more fixed electrodes housed in the reference vacuum cavity. When the pressure on the Px side of the diaphragm is higher than the pressure in the reference vacuum cavity, the diaphragm deflects in the direction of the fixed electrode (or electrodes), which increases the measured capacitance. As the pressure on the Px side of the diaphragm decreases, the pressure differential across the diaphragm diminishes and the diaphragm moves away from the fixed electrode (or electrodes) in the reference vacuum cavity, which reduces the measured capacitance.
As the pressure on the Px side of the diaphragm approaches the pressure in the reference vacuum cavity, the pressure differential across the diaphragm becomes sufficiently small as to be considered as the “zero point” for the CDG. This fixed zero point is established during the calibration of the CDG and is used as a reference in subsequent pressure measurements.
CDGs are commonly used to the measure pressure in vacuum chambers in which thin or thick films of material are deposited on a substrate. One common example of usage is to measure pressure during the deposition of materials onto the surface of silicon wafers during the fabrication of semiconductor devices. CDGs are quite useful in vacuum deposition processes that utilize multiple gasses because capacitance diaphragm gauges are highly accurate and are able to measure absolute pressure independent of gas composition.
The accuracy of the measurement of pressure by a CDG can be negatively impacted by several factors, one of which is a change in the orientation of the CDG with respect to a conventional orientation in which an X-Y plane of the CDG is parallel to the surface of the earth and the Z axis of the CDG is normal to the surface of the earth. In particular, a CDG is conventionally calibrated with the CDG in a known orientation. When the CDG is installed in a system with the CDG in a different orientation, the gravitational effects on the diaphragm may cause the diaphragm to have a biased deflection toward or away from the fixed electrode within the CDG. This biased deflection can result in an offset in the pressure reading.
The magnitude of the error caused by the misalignment of the axis of the CDG diaphragm with that of gravitational forces differs significantly for different mounting orientations. Historically, the gravitational effects have been crudely dealt with by the user re-zeroing the CDG after installation in the user's system. However, this is not a satisfactory solution for many users who want to be able to replace an existing CDG with a new CDG without having to recalibrate each time a CDG is replaced.